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New Scientist Live

Science: Viral burglars use chemical keys to break into cells

By RUTH LINTON

BIOLOGISTS in the US have observed how viruses spread through a plant
by breaking into its communication system. The viruses use chemical ‘keys’
to unlock the channels along which chemicals pass between cells. An understanding
of the mechanism may enable scientists to develop new ways of protecting
plants against viral infection, which causes huge amounts of damage and
loss of crops worldwide.

‘The viruses act as burglars,’ says William Lucas of the University
of California at Davis. ‘They have to break into each cell undetected. If
the plant cells detect an invader, they have numerous ways in which to prevent
the virus from spreading.’ Lucas and his colleagues from Davis and Washington
University, St Louis, have developed techniques to observe such viruses
in action.

To grow and develop normally, the cells in a plant must communicate
with each other. They do so along plasmodesmata, fine strands of cytoplasm,
which are laid down when the cells divide initially. It is along these channels
that chemicals, including sugars and hormones, pass from cell to cell. The
width of the channel determines the sizes of the molecules that can pass.

For many years, researchers have known that viruses can change the channels
in various ways. Now Lucas and his colleagues have found that the viruses
hijack the plant’s system of protein expression to manufacture chemical
keys, proteins that widen the plasmodesmata. The virus can then slip through
undetected and replicate.

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Lucas and his colleagues took RNA from the tobacco mosaic virus and
inserted it into a tobacco plant. By this means, they produced plants in
which the plasmodesmata remained wide open all the time. Using new techniques,
they were able to watch molecules move from cell to cell and to measure
how far the plasmodesmata open. Until now, studying molecular activity inside
living plant cells has been impossible because the active part of the cell,
the cytoplasm, occupies only a very thin layer inside the cell wall.

Lucas and his colleagues used a micro-pipette to release a tiny drop
of a specially designed liposome. This carried a small amount of fluorescent
marker inside the cell. The liposome fused with the plant cell, allowing
its contents to be released.

The researchers used a camera that can detect very faint light and translate
it into electrical signals which are then amplified a hundred million times.
With this equipment, they built up a colour computer image of molecular
activity inside the living plant as it took place. By attaching molecules
of known size to the fluorescent marker, they could measure how wide the
channels between cells open.

If scientists can discover more about the way in which these viral burglars
make their chemical keys, it should be possible to change plants so that
the chemical locks cannot be picked; in effect, engineering virus-proof
plants. Also, the new techniques developed to study the molecular activity
inside plants will enable researchers to study normal plants.